Highly polishable, highly thermally conductive silicon carbide

ABSTRACT

Silicon carbide is produced by chemical vapor deposition at temperatures from 1340°-1380° C., deposition chamber pressures of 180-200 torr, H 2  /methyltrichlorosilane ratio of 4-10 and deposition rate of 1-2 μm/min. Furthermore, H 2  supplied as a part of the gas stream contains less than about 1 part per million (ppm) O 2  gas, and various means are provided to exclude particulate material from the deposition chamber. The silicon carbide is polishable to &lt;5 Å RMS as measured on a Talystep mechanical profiler and has a thermal conductivity of at least about 300 W/mk. The silicon carbide is particularly suitable for applications where high polishability and thermal conductivity is desired, such as hard disc drives and read/write heads of head-disc assemblies, and also optical apparatus which require a very high polish.

This is a continuation-in-part of application Ser. No. 07/923,077 filedJul. 31, 1992, now abandoned.

The present invention is directed to highly polishable, highly thermallyconductive silicon carbide, production of such silicon carbide bychemical vapor deposition, and applications of such silicon carbide,particularly hard disc drives and read/write heads for head-discassemblies.

BACKGROUND OF THE INVENTION

There exists a need for improved substrates for component parts ofhead-disc assemblies (also known as "Winchester drives"), particularlysubstrates for hard discs and read/write heads.

Currently, the most widely used substrate material for hard discs andread/write heads is aluminum or aluminum alloys. To produce a hard-discfrom aluminum or an aluminum alloy, an aluminum or aluminum alloysubstrate is blanked, thermal flattened, sized and chamfered anddiamond-turned to a surface roughness of about 250 Å RMS. The blank isthen chemically pretreated to remove aluminum oxide prior to coatingwith an undercoat layer. The undercoat layer is typically electrolessnickel/phosphorous containing sufficient phosphorous so as to benon-magnetic. The nickel/phosphorous layer is polished and thentextured. To the undercoat is then applied a magnetic coating, typicallya cobalt/phosphorous alloy. Finally, a protective overcoat is applied,e.g., sputtered carbon. Additional layers and alternative procedures maybe used, depending upon the manufacturer.

There is a continuing desire to achieve higher memory capacity on discs,including higher storage (bit) density and track density. Aluminum andaluminum alloy substrates have inherent limitations with respect toachieving higher storage and track density; hence, there is a desire forsubstitute materials. Among aluminum's and aluminum alloys' limitationswith respect to their use as a substrate where high storage density isrequired are low elastic modulus, high coefficient of thermal expansion,and low Knoop hardness. Furthermore, aluminum exhibits poor chemicalresistance, oxidation resistance, thermal stability and polishability.The poor polishability (limited to about 100 Å RMS) necessitates thenickel/phosphorous undercoat.

Several materials have been considered as alternatives to aluminum, asfor example, as discussed in U.S. Pat. No. 4,808,463, the teachings ofwhich are incorporated herein by reference.

Glasses have certain advantages relative to aluminum and aluminumalloys, e.g., very low coefficient of thermal expansion, but also havelimitations as substrate for magnetic recording components; inparticular, being electrically non-conductive, and having very lowthermal conductivity (watts per meter per °Kelvin (W/mK)).

Silicon carbide has a number of inherent properties which suggest itsuse as substrates for magnetic recording components, in particular, highspecific stiffness, strength, hardness, thermal conductivity, lowthermal expansion and chemical and oxidation resistance and iselectrically conductive.

Silicon carbide produced by sintering (e.g., Japanese patent document88-128885/19, Sep. 12, 1986 Hitachi KK) and reaction bonding (e.g., U.S.Pat. No. 4,598,017, the teachings of which are incorporated herein byreference) have been tested for use in magnetic recording media. Toproduce sintered silicon carbide, powdered silicon carbide is admixedwith sintering aids and compacted using heat and pressure. The need forsintering aids results in the sintered silicon carbide having voids(sintered silicon carbide generally has a density of no greater thanabout 90% of theoretical density), impurities (residues of the sinteringaids), and has relatively loosely bound crystals. Accordingly, sinteredsilicon carbide must be coated if it is to be used as a substrate forrecording media. In reaction bonded silicon carbide, silicon fills thevoids; thus reaction bonded silicon carbide is heterogeneous, thesilicon detracting from desirable properties of silicon carbide. Thebest polishability of reaction-bonded SiC is about 30-50 Å RMS; sinteredSiC is even less polishable.

It has been earlier proposed by others to evaluate silicon carbide whichis deposited by chemical vapor deposition (CVD) as a candidate materialfor substrates of magnetic recording components. CVD-produced siliconcarbide can closely approach 100% of theoretical density, has a tightlybound granular structure, and good polishability. Because CVD-producedSiC is highly polishable, it can be directly coated with a magneticcoating media, unlike aluminum or aluminum alloys which requireapplication of a nickel/phosphorous undercoat prior to application ofthe magnetic coat.

Silicon carbide is generally deposited by CVD from a gaseous mixture ofmethyltrichlorosilane (MTS), H₂, and generally an inert or non-reactivegas such as argon, helium or nitrogen, argon being preferred.Free-standing SiC is typically pyrolytically deposited on a mandrel,such as a graphite mandrel, from which it is removable. The MTS is thepreferred source of both the Si and C and provides these instoichiometric (1:1) ratios. The H₂ scavenges Cl, producing HCl. Theinert or non-reactive gas acts as a carrier gas for MTS (which is liquidat ambient temperatures); can be varied to adjust velocity of gas flowthrough the furnace as is necessary to sweep reaction product, such asHCl, from the deposited SiC; and acts as a diluent, preventing gas-phasereactions which might introduce impurities into the SiC.

CVD production of free-standing SiC material by providing a furnacehaving a deposition chamber and a mandrel therein and pyrolyticallydepositing SiC on the mandrel are described, for example, in U.S. Pat.Nos. 4,900,374; 4,997,678; and 5,071,596, the teachings of these patentsbeing incorporated herein by reference.

The present invention is directed to CVD-deposited silicon carbideparticularly suitable for magnetic recording media, particularlycomponents of a head-disc assembly (HDA). The CVD produced in accordancewith the invention has a combination of excellent thermal conductivityand high polishability not heretofore achieved in free-standing siliconcarbide.

The fabrication of HDAs, including hard discs and read/write heads aredescribed, for example, in R. W. Wood, "Magnetic Recording Systems",Proc. of the IEEE, 74(11), 1557-1569 (1986); C. Warren, "Rigid-diskDrives: Capacity, Performance Mount as Size Shrinks", Electronic Design,Apr. 28, 1983, pp. 139-150; Ivan Flores, "Chapter 5: External Storage"in The Professional Microcomputer Handbook (Van Nostrand Reinhold Co.,New York, N.Y., 1986) pp. 111-151; U.S. Pat. No. 4,647,494; and articlesin IBM Disc Storage Technology February 1980, including "Film HeadDevelopment" by D. A. Thompson et al. (pp. 3-5) and "IBM 3370 Film HeadDesign and Fabrication" by R. E. Jones, Jr. (pp. 6-9), the teachings ofeach of these being incorporated herein by reference.

High polishability is a very important attribute of a substrate formagnetic recording components of HDAs or the like. The a real densitystorage of a magnetic disc is determined by the size of the individualmagnetic domains which can be achieved. The magnetic domain (or cell)size is directly related to the head fly height, i.e., the distance orgap by which the read/write head "flys" above the hard disc. The lowerthe fly height, the smaller the magnetic domains that can be achieved.Therefore, lower fly height translates into higher a real density. Flyheight is controlled by a number of factors, surface smoothness beingone of them. A limiting factor of the gap between the head and the disc(fly height) appears to be surface roughness. Surface roughness producesturbulence in the "air" gap between the disc and head, causing the headto crash into the disc if the gap is too narrow. Furthermore, lower flyheights translate to higher track density on the hard disc, increasingthe overall memory storage capacity of the disc.

SUMMARY OF THE INVENTION

In accordance with the invention, free-standing silicon carbide isproduced by chemical vapor deposition which has very high polishability,i.e., about 5 Å RMS or less as measured on a Talystep mechanical contactprofiler and even as polishable to about 1 Å RMS or less. TheCVD-deposited SiC has very high thermal conductivity, i.e., at leastabout 300 W/mK, a property advantageous in dissipating heat. The SiC isdeposited at very specific deposition conditions usingmethyltrichlorosilane (MTS), H₂ gas and usually an inert carrier gas,which conditions include: a deposition chamber pressure of between about180 and about 220 torr, a deposition chamber temperature of betweenabout 1340° and about 1380° C., a deposition rate of between about 1.0and about 2.0 μm/min., and an H₂ /MTS gas partial pressure flow ratio ofbetween about 4 and about 10. Furthermore, H₂ supplied as a part of thegas stream, is purified so that it contains less than about 1 part permillion (ppm) O₂ gas, and various means are provided to excludeparticulate contaminant material from the deposition chamber. TheCVD-deposited SiC is machined to its end-use configuration, e.g., a harddisc or a read/write head, and highly polished on an appropriate surfaceor surfaces. To make a hard-disc, the highly polished surfaces(hard-discs are generally two-sided) are coated with magnetic recordingmedia, e.g., a cobalt/phosphorous alloy magnetic media, and, generally,further coated with a protective overcoating, e.g., sputtered carbon. Ina read/write head, read/write circuitry are fabricated on a highlypolished surface of the silicon carbide, e.g., by photolithography.Additional layers may be included by different manufacturers. Forexample, there may be benefits to initially coating the SiC substratewith Al₂ O₃ prior to applying the magnetic recording medium layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view of a furnace used toproduce SiC in accordance with the method of the present invention.

FIG. 2 is an enlarged diagrammatic cross-sectional view of an injectornozzle extending to the deposition chamber of the furnace of FIG. 1.

FIG. 3 is a further enlarged diagrammatic cross-sectional view of theinjector nozzle of FIG. 2.

FIG. 4 is a diagrammatic illustration of apparatus used in the hydrogenfeed line to provide substantially oxygen-free hydrogen to the furnace.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

The present invention provides free-standing, cubic (β) phase SiC whichis highly polishable, i.e., about 5 Å RMS or less, preferable about 3 ÅRMS or less, and most preferably about 1 Å RMS or less as measured on aTalystep mechanical contact profiler. Herein, unless otherwise noted,polishability values are as measured on a Talystep mechanical contactprofiler. The value of surface roughness (polishability) may varysignificantly depending upon the measurement technique. For example,surfaces measured to be 1 Å RMS on a Talystep mechanical contactprofiler would measure lower on a Zygo heterodyne profiler and larger onan atomic force microscope. A more detailed discussion of surfaceroughness measurements is found in J. M. Bennett, V. Elings and K.Kjoller, Optics & Photonics 2 (5) May 1991, pp. 14-18; J. M. Bennett andS. H. Dancy, Appl Opt. 20, 1984, pp. 1785-1892. Free-standing SiC is tobe distinguished from SiC films which are supported by and inseparablefrom a backing material in that free-standing siC can be unsupported andpolished. Generally, the thinnest free-standing SiC which can bepolished is between 5/1000 and 10/1000 of an inch (0.127-0.244 mm). (Itmay well be that some SiC thin films on extremely smooth substrates canbe highly polished to an extent approaching or equaling that of theunderlying substrate.)

As mentioned above, a high degree of polishability is desired in asubstrate for magnetic recording components in order to achieve low flyheights between the read/write head and the hard disc in a head-discassembly (HDA) and consequential high memory storage capacity. The highdegree of polishability is also desirable for optical apparatus, e.g.,SiC mirrors. High thermal conductivity is highly desirable fordissipating heat in components of HDAs.

Even though the polished surface may be subsequently coated with severalcoats of material, including magnetic recording media, an overcoat andoptional other layers, any surface irregularities in the polishedsurface tend to be imparted to the subsequent layers, often inexaggerated form. A high polish also permits superior surface cleaningprior to subsequent coating, resulting in better bonding of thesubsequent coating. This results in fewer rejected components.

A high thermal conductivity in magnetic recording components is highlydesirable from the standpoint of dissipating heat which builds up duringoperation.

Thermal conductivity is strongly dependent on the grain size and purityof the material, i.e., the thermal conductivity increases withincreasing grain size and low impurity concentration along the grainboundaries. The grain size and, therefore, the thermal conductivity, iscontrolled by the deposition temperature, pressure, and gas flow rates.For example, under conditions of high temperature and low MTS flow rate(which results in low MTS partial pressure), the grain size increases.As temperature is lowered and the MTS flow increases, the grain sizedecreases. SiC produced in accordance with the invention has a thermalconductivity of about 300 W/mK or above.

The polishability depends upon several factors. The SiC must bevoid-free, of high purity (no second phase material) and stoichiometric(equal number of Si and C atoms) in order to obtain highly polishedsurfaces. The stoichiometry depends upon the deposition temperature,pressure and H₂ /MTS ratio. Also, the deposition rate must be relativelylow in order to-obtain a void-free material. The deposition rate iscontrolled by the deposition temperature, pressure and gas flow rates.The deposition rate increases with increasing temperature, pressure andMTS flow rate.

Herein, CVD-deposited, free-standing SiC having very high polishabilityand high thermal conductivity is deposited under very specificconditions. Certain parameters, such as individual gas-flow rates willvary according to the size and design of the particular CVD depositionfurnace. It is found, however, that CVD-deposited, free-standing SiChaving polishability of about 5 Å RMS or less, preferably about 3 Å RMSor less and most preferably about 1 Å RMS or less and thermalconductivity of at least about 300 W/mK is deposited under a veryspecific set of reaction conditions using the gas mixture of at leastmethyltrichlorosilane (MTS) and H₂ ; in particular: a furnace pressureof between about 180 and about 220 torr, a deposition temperature ofbetween about 1340° and about 1380° C., preferably between about 1340°C. and about 1370° C., and most preferably about 1350° C., depositionrate of between about 1.0 and about 2.0 μm/min., and an H₂ /MTS gaspartial pressure flow ratio of between about 4 and about 10.Furthermore, H₂ supplied as a part of the gas stream is purified so thatit contains less than about 1 part per million (ppm) O₂ gas, and variousmeans are provided to exclude particulate contaminant material from thedeposition chamber. The gas mixture almost always includes an inert gas,preferably argon, as a carrier for MTS and in an amount to adjustaerodynamic properties of the gas stream. However, it is conceivablethat the process might be run using only MTS carried by H.sub. 2.Although these specific deposition parameters may fall within broadranges described in afore-mentioned U.S. patents, it is demonstratedhereinafter, that the very specific set of deposition parameters setforth above achieve the combination of very high polishability and veryhigh thermal conductivity not found in any actual deposition shown inthe prior art.

With respect to "deposition chamber temperature," what is meant hereinis the temperature as determined in the deposition chamber in closeproximity to the mandrel on which the silicon carbide is beingdeposited, although the actual mandrel temperature may be slightlydifferent. Experience has shown that temperatures at different locationsrelative to, but each closely adjacent the mandrel seldom vary more thanabout 5° C., and it is assumed that the actual deposition temperature atthe mandrel is within 10° C. of the probed temperature.

Thermal conductivity is believed to be relatively consistent, regardlessof method of measurement. Herein, thermal conductivity is the thermalconductivity as measured by flash lamp techniques.

Unless otherwise stated, all properties of SiC are measured herein atroom temperature, i.e., 20°-28° C. (293°-301° K.).

Free-standing SiC is generally at least 5/1000 in thick (0.127 mm). Forpurposes of making hard discs and read/write heads, SiC is generallydeposited to a thickness of at least about 2-3 mm. The SiC substrate maybe partially configured according to the particular mandrel on which theSiC is deposited, but is generally further configured and/or finished bymachine tooling following deposition. The substrate is polished on asurface or surfaces according to the requirements of the component partby conventional means, such as lapping using pitch and progressivelyfiner diamond abrasive.

In producing a magnetic recording component, e.g., a hard disc or aread/write head, after polishing a surface(s) to the desired degree,e.g., about 5 Å RMS or less, preferably about 3 Å RMS or less and mostpreferably about 1 Å RMS or less, magnetic recording media is depositeddirectly on the polished surface(s) by conventional means, such aselectroless plating or a sputtering process. Magnetic recording mediamay be selected from a variety of magnetic materials or alloys such asGdCo, coPtCr, and CrV-CoPtCr. Cobalt/phosphorous alloy is a currentlypreferred magnetic recording media. Typically, the magnetic recordingmedia layer thickness is 500-1000 Å. To protect the magnetic recordingmedia layer, it is preferred that the magnetic recording media layer beovercoated with a protective layer, such as sputtered carbon, to athickness of 200-500 Å. The component may be further surface-lubricatedto improve stiction/friction performance.

Properties of SiC produced in accordance with the invention andadvantages with respect to magnetic recording components are summarizedin the table below:

    ______________________________________                                        PROPERTY        ADVANTAGE                                                     ______________________________________                                        High Polishability                                                                            Low fly-heights                                                               High storage density                                          High Specific Stiffness                                                                       Low fly-heights                                                               Thin discs                                                                    Low weight                                                    High Thermal Conductivity                                                                     Heat dispersion                                                               Low distortion                                                Chemical, Oxidation                                                                           Reliability, low error rates                                  Resistance                                                                    Hardness (Scratch Resistant)                                                                  High yields                                                                   Low contact damage                                                            Durability                                                    Thermal Stability                                                                             Low fly-heights                                               Electrical Conductivity                                                                       No static charge build-up                                     High Yield Strength                                                                           Low breakage                                                                  High in-process yield                                         Low Thermal Expansion                                                                         Low distortion from temp. change                              Dimensional Stability                                                                         No "bowing" during cutting                                    (Low Stress)    (heads)                                                       Good Machinability                                                                            No "chipping" sharp edges                                                     Minimal edge roll-off.                                        ______________________________________                                    

To achieve very high thermal conductivity, i.e., about 300 W/mK orabove, along with very high polishability, it has been found necessaryto substantially exclude from the deposition chamber both oxygen, whichtends to cause deposition of carbon inclusions, and particulates, whichmay become inclusions in the deposited CVD. The main source of oxygen isgenerally the hydrogen gas, and means are provided to reduce to andmaintain the presence of oxygen in the hydrogen gas stream to less thanabout 1 ppm. Likewise, means are provided for maintaining the depositionchamber substantially free of particulates or other possible inclusions.Applicants' approach to substantially eliminating oxygen andparticulates is described below with respect to FIGS. 1-4. It is to beunderstood that other means may be employed to eliminate oxygen andparticulates, depending upon the apparatus used. The figures onlyillustrate those portions of the apparatus which are modified forreducing particulates, as general CVD apparatus for depositing siliconcarbide are known, e.g., in patents referenced above.

Illustrated in FIG. 1 is a diagrammatic cross-sectional illustration ofa 0.5-m CVD-SiC furnace 10 adapted by Applicants for depositing theirhighly polishable, highly thermal conductive SiC. The apparatus includesan outer wall or housing comprising a cylindrical sidewall 11, a topplate 12 and a bottom plate 14. An inner wall or housing comprises acylindrical sidewall (or isolation tube) 16, a top cover 18 and a bottomcover 20; the inner wall defines the inner space which is the depositionchamber 22. Surrounding the isolation tube 16 is a side heater 24; belowthe bottom cover 20 is a bottom heater 26. Each of the heaters 24, 26are of the electrical resistive type, e.g., being formed of graphiteglow bars.

Interposed between the inner housing and outer housing and surroundingthe inner housing and heaters 24, 26 are batts of insulating material 28which are usually formed of fibrous material, e.g., carbon fibers. Aswill be explained in greater detail in respect to FIG. 2, these batts 28are a source of particulate material, for example, during furnacedisassembly; and means are provided for eliminating the batts as asource of particulate material to the deposition chamber. Injector means30 provide for entry of gas through the upper end of the furnace an tothe deposition chamber; in the illustrated furnace 10, three injectors30 are illustrated. As will be explained in greater detail with respectto FIG. 2, the injectors are a potential source of particulates,including relatively large particulates, and the injectors 30 in theillustrated furnace have been modified from previous injector design toeliminate the injectors 30 as a source of particulate material in thedeposition chamber 22. An exhaust tube 32 communicates with the lowerend of the deposition chamber 22.

In the deposition chamber 22 interposed in the flow of gas, is a baffleplate 34 which is supported just above the bottom cover 22 in ahorizontal orientation. Supported above the baffle plate 34, also in ahorizontal orientation, is the mandrel 36 on which the bulk CVD SiC isdeposited. Deposition chamber 22 temperature is monitored bythermoconductors 38 and optical pyrometers 40.

Although not illustrated in detail herein, the furnace is disassembledthrough the bottom between each run. During disassembly, there is atendency for particulates breaking off from the carbon felt insulation28 to contaminate the deposition chamber 22. To prevent this, protectiveceramic blankets or covers are provided over the carbon felt batts 28.The cover means illustrated in FIG. 2 comprise cover sheets of nonporousceramic gasket material 42, for example graphite, such as that soldunder the tradename "Graffoil", and carbon sleeves 44 capping orificesthrough the batts 28.

In previous furnace designs, the injectors 30 have been the source ofparticulate material which drops onto the mandrel 36 below and the SiCbeing deposited thereon. As diagrammatically illustrated in FIG. 3, eachinjector 30 is comprised of three concentric tubes which define annularpassageways 56 and 58 and a central passageway 60. The centralpassageway 60 is the gas passageway and is open at its lower end tocommunicate with the deposition chamber 22. An annular cap 62 extendsbetween the outer and inner tubes 50, 54, sealing the lower end of theannular passageways 56, 58 to each other. The lower end of the middletube 52 is spaced above the annular cap 62 to provide a gap 64 thatcommunicates the annular passageways 56, 58. A coolant, usually water,is pumped downward through the inner annular passageway 58 and returnsthrough the outer annular passageway 56. This coolant preventsdeposition from occurring in the gas flow passageway 60 in the heatedregion where it communicates with the deposition chamber 22 by keepingthe passageway 60 below the deposition temperature.

It is to be appreciated that the injector tube 30 must be movablerelative to at least the top plate or cover 12 of the outer housing orthe top cover 18 of the inner housing because of the differenttemperatures of each of the housings and the resulting difference inexpansion and contraction during furnace operation. In previous furnacedesigns (not shown) the lower end of the injector tube extended downwardbelow the top cover 18 of the inner housing and into the depositionchamber 22 to ensure continual communication between the central passageof the injector and the deposition chamber. This was found to beproblematic in that the cooler injector tended to promote condensationand film build-up thereon, which deposition might flake off and fallonto the mandrel 36 therebelow and become large particulate inclusionsin the SiC being deposited thereon.

To overcome this problem, the injector tubes 30 have been adapted toextend downward to, but not into, the deposition chamber 22, providingcontinuous communication with the deposition chamber, while beingrelatively slidable relative to the top plate 12 of the outerhousing.Illustrated in FIG. 2 is applicants' design for injector apparatus whichprovides for the injector tube 30 extending to, but not into, thedeposition chamber 22. An inlet opening 70 in the top cover 18 of theinner housing is shaped to provide an annular ledge 72. Seated in theopening 70 on the ledge 72 is a carbon washer 74 of appropriate interiordiameter that the injection tube 30 seats thereontop. To facilitateseating of the injector tube 30 on the washer 72 during furnaceassembly, a tubular carbon guide 76 having a tapered upper end 78 alsois seated in the opening 70 on the washer 72. The injector tube 30extends upwards through openings 80, 82 in the insulation 28 and topplate 12.

It is important that the openings in the outer housing, including theopening 82, through which the injector tube 30 extends, be sealed.Accordingly, a flanged sleeve 84 is provided at the opening 82, fixed tothe top plate 12. The injector tube 30 is vertically movable through theflanged sleeve 84 to accommodate differential thermal expansion of theinner and outer housings. A coupling 86 is provided around the injectortube 30 at a location above the top plate 12. Providing a seal betweenthe coupling 86 and the flanged sleeve 34 is a bellows 90 thataccommodates relative vertical movement between the top plate 12 and theinjector tube 30. To provide for disassembly and assembly, theillustrated bellows provides for attachment and separation 88 at twopoints; 0-rings 88 at these points provide a gas-tight seal duringoperation.

As discussed briefly above, another source of inclusions which detractfrom uniform deposition of SiC is the presence of oxygen. In examinationof previous depositions, small carbon inclusions could be found. Also,it was noted in SiC depositions that bands of carbon inclusions tendedto form when hydrogen cylinders were changed, indicating that oxygenmight be the culprit. To avoid changing hydrogen cylinders, sufficientlylarge hydrogen cylinders are now being employed to supply an entiredeposition run. Nevertheless, commercially available hydrogen sourcestend to contain amounts of oxygen which, though very minor, aresignificant with respect to producing carbon inclusions in CVD-SiC.Accordingly, to produce the SiC of the present invention, it was foundnecessary to purify the hydrogen gas, particularly to substantiallyeliminate trace oxygen from the hydrogen.

Illustrated in FIG. 4 is a schematic of a hydrogen gas line 99 whichapplicants have modified to eliminate oxygen and some other gaseousimpurities. The hydrogen gas line confluences with a flow ofargon-carried MTS (not shown), and the combined flow of argon, hydrogenand MTS is introduced into the deposition chamber 22 through theinjectors 30. The gas line includes the hydrogen source 100. Downstreamfrom the hydrogen source 100 is a catalytic purifier 102, such asResource Systems, Inc., Model RCP-500, which converts oxygen to water.Downstream of the catalytic purifier 102 is a molecular sieve dryer 104,such as Resource Systems, Inc., Model MSD-1000, which traps the water.The system reduces the oxygen content of the hydrogen gas stream tobelow 1 ppm. This system is also capable of removing carbon dioxide,ammonia and other undesirable polar molecules. Although this systemrepresents Applicants' current preferred method of removing O₂ from theH₂ gas stream, other means, such as a gettering system, might beemployed for this purpose.

The invention will now be described in greater detail by way of specificexamples.

EXAMPLE 1

Conditions were optimized for a 0.5-m and a 1.5-m production furnace ofMorton Advanced Materials, Woburn Mass., which conditions are given inthe table below. It is to be appreciated that chemical vapor depositionfurnaces, as are those used in this example, are custom manufactured,and that specific conditions of flow rate may vary from furnace tofurnace. To produce CVD-deposited SiC in accordance to the invention,temperature, pressure, H₂ /MTS ratio and deposition rate should bewithin the ranges set forth below, regardless of specific furnacedesign.

    ______________________________________                                        Process Parameter                                                                             0.5-m Furnace    1.5-m Furnace                                ______________________________________                                        Furnace Pressure                                                                              200    torr      200  torr                                    Deposition Temperature                                                                        1350°                                                                         C.        1350°                                                                       C.                                      Gas Flow Rates:                                                               H.sub.2         28     slpm      140  slpm                                    Ar              72     slpm      550  slpm                                    MTS             5.6    slpm      28   slpm                                    Gas Partial Pressures:                                                        H.sub.2         53     torr      39   torr                                    Ar              136    torr      153  torr                                    MTS             10.6   torr      7.8  torr                                    Deposition Rate:                                                                              1.5    μm/min 1.9  μm/min                               H.sub.2 /MTS Gas Flow Ratio:                                                                  5                5                                            ______________________________________                                    

EXAMPLE 2

Using the 0.5-m Furnace of Example 1, CVD-SiC is deposited in accordancewith the conditions described in U.S. Pat. No. 4,990,374 lines 30-36,i.e.: deposition temperature 1300° C.; furnace pressure 200 torr; gaspartial pressures Ar 68 torr, H₂ 102 torr and MTS 30 torr. The followingtable compares properties of the SiC deposited using the conditions inExample 1 with the SiC deposited in accordance with U.S. Pat. No.4,990,374. (Values specified room temperature measurements values(20°-25° C.).

    ______________________________________                                        Property     Example 1 Conditions                                                                          4,990,374 Cond.                                  ______________________________________                                        Crystal Structure                                                                          FCC, polycrystalline                                                                          FCC, polycryst.                                  Grain Size (μm)                                                                         17              8                                                Density (g/cm.sup.-3)                                                                      3.21            3.21                                             Hardness (Knoop,                                                                           2500            2500                                             g load)                                                                       Chemical Purity                                                                            99.999% SiC     Unknown                                          Flexural Strength,                                                                         430             490                                              4-point (MPa)                                                                 Elastic Modulus                                                                            466             Unknown                                          Coefficient of                                                                             2.2 × 10.sup.-6                                                                         2.2 × 10.sup.-6                            Thermal Expansion                                                             (K.sup.-1)                                                                    Thermal                                                                       Conductivity >300 W/mK       100-145 W/mK                                     Polishability                                                                              <1 Å RMS    ˜10 Å RMS                              ______________________________________                                    

Of the above properties, polishability of the SiC deposited inaccordance with the invention is of great significance for substratesfor magnetic recording components and also optical components requiringa very high degree of polishability. Improved thermal conductivity isalso of important significance for substrates for magnetic recordingcomponents.

EXAMPLE 3

In a 70 hr. run, CVD-SiC was deposited in a 0.5 mm furnace under thefollowing conditions:

    ______________________________________                                        Furnace Pressure      200    torr                                             Deposition Temperature                                                                              1350°                                                                         C.                                               Gas Flow Rates:                                                               H.sub.2               28     slpm                                             Ar                    77     slpm                                             MTS                   5.9    slpm                                             Gas Partial Pressures:                                                        H.sub.2               50     torr                                             Ar                    139    torr                                             MTS                   10.6   torr                                             Deposition Rate       1.5    μm/min                                        H.sub.2 /MTS Gas Flow Ratio                                                                         4.7                                                     ______________________________________                                    

The CVD-SiC has a thermal conductivity of 304.9 W/mK at 28° C. (301° K.)and is polished to <1 Å RMS.

While the invention has been described in terms of certain preferredembodiments, modifications obvious to one with ordinary skill in the artmay be made without departing from the scope of the invention.

Various features of the invention are set forth in the following claims.

What is claimed is:
 1. Free-standing polycrystalline B, chemical vapordeposited silicon carbide having a thermal conductivity of at leastabout 300 W/mK at room temperature.
 2. Silicon carbide according toclaim 1 having at least one surface polished to about 5 Å RMS asmeasured on a Talystep mechanical contact profiler.
 3. Silicon carbideaccording to claim 2 wherein said surface is polished to about 3 Å RMSor less.
 4. Silicon carbide in accordance with claim 2 wherein saidsurface is polished to about 1 Å RMS or less.